Applicant claims, under 35 U.S.C. xc2xa7119, the benefit of priority of the filing date of Sep. 30, 1999 of a German patent application Serial Number 199 46 931.9, filed on the aforementioned date, the entire contents of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method for operating an eddy current sensor, wherein a first magnetic flux, which is essentially directed vertically with respect to a moved body, is generated by at least one device. The invention also relates to an eddy current sensor for executing the method.
2. Discussion of Related Art
Velocity or acceleration sensors based on the principle of measuring eddy currents are also called Ferraris or eddy current sensors. Permanent magnets induce voltages U in a moving, electrically conducting non-ferromagnetic body as a function of the velocity v, which cause eddy currents I as a function of the velocity. Changes in velocity dv/dt change the magnetic field generated by the eddy currents, because of which voltages U(dv/dt) are induced in detector coils.
Such an eddy current detector is known from DE 37 30 841 A1, which can be used as a speedometer or accelerometer. DE 37 30 841 A1 corresponds to U.S. Pat. No. 4,751,459, the entire contents of which are incorporated herein by reference. It contains a non-magnetic, electrically conductive body, whose velocity or change in velocity can be measured. A constant magnetic field is generated essentially vertically with respect to the direction of movement and leads to eddy currents in the moved body, which in turn generate an eddy current magnetic field. When used as a speedometer, the flow density of the eddy current field is measured by a Hall effect sensor, whose output signal represents the velocity. When used as an accelerometer, the change of the flow density of the eddy current field over time is measured by a coil, whose output signal is proportional to the acceleration. To concentrate the eddy current flow and to reduce interference effects, a separate magnetic circuit is used for the coil or the Hall effect sensor.
It is disadvantageous here that the moved body can move at very different velocities and that the current strength of the eddy current rises with the velocity. Since, because of the electrical resistance of the conductive moved body, the eddy current leads to thermal heating, the moved body can become very hot at high velocities. This also affects the eddy current field, which leads to inaccuracies in measurement.
A method and an arrangement is known from U.S. Pat. No. 4,893,079, the entire contents of which are incorporated herein by reference, wherein the effects of the temperature in an eddy current detector are corrected. An eddy current detector, in which measurement errors caused by a temperature change are compensated, is used in connection with an arrangement for measuring physical parameters of conductive materials. Circuitry is used for this, which connects the primary coils and reference coils of the eddy current detector either with an oscilloscope for displaying the eddy current, or with an ohmmeter for displaying the electrical resistance. A resistance change can be measured on the basis of a temperature change by this and can be taken into account in the determination of the eddy current.
Although it is possible by this to essentially compensate measuring inaccuracies in an eddy current sensor caused by temperature fluctuations, the disadvantage remains that the eddy current detector itself leads to warming because of the induced eddy currents and thereby worsens the measurement results. If the eddy current detector is moved rapidly, this effect is increased. However, it is not disclosed how warming, which is unavoidable because of the basic physical conditions in the measurement of eddy currents, can be reduced.
A rotational position measuring system is known from EP 0 661 543 B1, wherein the rotational acceleration is also measured. Two signal transmitters are connected in a torsion-proof manner with each other, and a signal detection unit is assigned to each signal transmitter. A first one of the two signal transmitters is produced by means of an optical or inductive graduation, which is scanned by a signal detection unit by an optical or inductive scanning head. A second signal transmitter includes an electrically conducting disk, through which a magnetic flux flows in a vertical direction. This magnetic flux can be generated by suitable magnets. If the disk is moved in relation to the magnet, eddy currents are created, which in turn generate a magnetic field. The change in the magnetic field is qualitatively detected by a signal detection unit, so that the measured value detected in the signal detection unit represents a value of the acceleration. The two signal transmitters are either arranged, each in the form of a separate disk, on a common torsion-proof shaft, or for position measuring the first signal transmitter is arranged directly on the edge of the second signal transmitter for acceleration measurement, because of which the disk diameter as a whole is increased.
In this case, it is disadvantageous that temperature problems are created at high numbers of revolution, since the magnetic flux must be of such a strength that an exact determination of the acceleration is also possible at slow accelerations and low numbers of revolution, which is only assured starting at a defined minimum value of the magnetic flux. However, this causes problems at high numbers of revolutions, since in that case the eddy currents caused in the second signal transmitter lead to very great heating of the signal transmitter. Since this greatly heated signal transmitter is made of metal in most cases, heating leads to a not inconsiderable expansion, which can result in a deformation of the first signal transmitter, if the latter is fastened directly on the outer edge of the second signal transmitter. The function of the first signal transmitter is negatively affected by this. If the first signal transmitter includes an optical graduation, which had been applied to glass, it could even be destroyed. No steps for preventing this heating are disclosed.
In summary, when an eddy current sensor is used as an acceleration detector, a problem arises in that a magnetic field of a large magnetic field strength is required for performing an accurate measurement at low velocities and slow accelerations. However, at large velocities and with great accelerations, large eddy currents are induced in the moved body, which lead to undesirably large heating.
It is therefore an object and advantage of the present invention to design a Ferraris or eddy current sensor in such a way that it is possible to counteract the impermissible heating. Moreover, it is intended to produce the eddy current sensor compactly and cost-effectively.
This object and advantage is attained by a method for operating an eddy current sensor that includes generating a first magnetic flux, which is essentially directed vertically with respect to a moved body, generating eddy currents in the moved body in response to the first magnetic flux, wherein the eddy currents generate an eddy field, inducing a voltage in a detector as a result of a change of the eddy field and superimposing a further adjustable magnetic flux on the first magnetic flux to form a resultant magnetic flux.
The above objective and advantage is attained by an eddy current sensor including an arrangement for generating a first magnetic flux, which is essentially directed vertically with respect to a moved body, a detector coil for detecting an eddy field in the moved body and a second arrangement which generates a further adjustable magnetic flux, which is superimposed on the first magnetic flux to form a resultant magnetic flux.
The Ferraris or eddy current sensor in accordance with the present invention, and the method executed with the aid of it have the advantage that the field strength of the magnetic field, by which eddy currents are caused in the moved body, can be adapted as a function of the velocity of the moved body. Thus, there is no longer a constant magnetic field, as in the prior art, instead it is changed in accordance with the present invention, in particular as a function of the velocity.
Modification coils are advantageously provided, whose magnetic field is superimposed on that of the permanent magnets, and whose current is impressed as a function of the velocity. By this it is possible as a function of the velocity of the moved body to always set the optimal field strength of the magnetic field with which an exact acceleration measurement is possible, but where the moved body is not overly heated. It is therefore possible to always set the optimal field strength of the constant magnetic field.
It is furthermore possible to make the permanent magnets for generating the constant magnetic field smaller, since the constant magnetic field can not only be weakened by the additional coils, but also strengthened. This makes possible considerable improvements in regard to the weight and volume of the eddy current sensor. Moreover, the manufacture of the eddy current sensor becomes more cost-effective because of this.
Further advantages, as well as details of the eddy current sensor in accordance with the present invention, as well as of the method in accordance with the invention, ensue from the following description of the exemplary embodiments by means of the drawings.